Electrolytic tinning process



Jllne 1l, 1963 l.. M. BERNlcK ETAL 3,093,555

ELECTROLYTIC TINNING PROCESS Filed June l, 1959 n Z -l' l? J3 13 J3 i" 12,14 g@ /JZ 14 .i4/g 4 12g@ 2/4. @H3000 s @[[Qugc qcng^ggousqocbog=b9ccoi /@ljllz L15 J5 J6/@J1 Z515 13 J3 J3 13 u f3 lll U lll f I .l a lll 2 3 .v .oa .og .1o .11 .12 .1a .14 .16 JNVENToRs.

'FREE "N (LBS/B9) feslLeMenz'C' Z/L'vecenZPPean-om Z/Z'QWCSL'ez/er @Mw @Mm @f5- United States Patent Office 3,093,555 Patented June 11, 19673 3,093,555 ELECTROLYTIC TINNING PROCESS Leslie M. Bernick, Calumet City, Ill., and Vincent P.

Pearson and William C. Sievert, Chesterton, Ind., as-

signors to Inland Steel Company, Chicago, lll., a corporation of Delaware Filed June 1, 1959, Ser. No. 817,256 6 Claims. (Cl. ZIM- 28) This invention relates generally to a process for-electroplating tin and, more particularly, to an improved continuous process for coating a strip of ferrous metal in an alkaline-type electrolytic tinning process to provide ditferentially tin-coated surfaces thereon.

The production of tin plate by continuousv electrolytic tinning involves the steps of passing a ribbon or strip of steel through a tank containing a solution of a tin salt and therein electrodepositing tin on the strip by providing within the tank a number of anodes, each connected with a source of electric current, for plating one side of the strip, and an equal number of anodes, each connected with a similar source of electric current, for plating the other side of the strip. After leaving the tinning tank, the freshly plated strip passes through a furnace or oven at a temperature suthciently high to fuse the tin in order to form a bright even coating on the steel strip, the latter step being known as flow brightening.

A more recent development in electrolytic tinning is the production of differentially coated tin plate wherein the electrolytic tinning step is so regulated that one side of the steel strip receives a relatively heavy coating of tin and the other side receives a relatively light-weight coating of tin. For purposes of manufacturing tin cans and the like, a relatively heavy, corrosion-resistant coating of tin is required only von the inside of the can. Thus, the use of differentially coated tin plate avoids an unnecessarily heavy external coating of tin, thereby resulting in important economies in the use of tin.

It has been customary to operate a continuous electrolytic tin coating process employing an alkaline tin plating bath by passing a steel strip at relatively high speed between the spaced anodes arranged within the plating bath. The speed at which the steel strip is moved in the alkaline plating bath may vary between about 400 feet per minute and 800 feet per minute, depending upon the thickness of the coating desired and the amount of current available at the anodes. Since there is generally only a limited amount of current available yfor any given tinning line, when a relatively thick coat of tin is required, it is necessary to reduce the rate at which the steel strip passes between the anodes as compared with the maximum rate at which the strip travels when a relativelyA light coat of tin is desired. ltis -thus evident that the maximum rate of travel of a steel strip being differentially coated must dependV on the current available `for plating the heavier side of the strip, and when operated at the maximum permissible rate for coating the heavier side, it will be necessary to reduce the amount of current supplied to the anodes depositing the thin coating of tin on the steel strip. p

In the production of diiferentially coated electrolytic tin plate with an alkaline-type electrolyte to provide, for

example, a trn coating weight of 0.25 pound per base box on one side of the strip and 1.00 pound per base box on the other side thereof, the maximum speedV of operation is limited by the amperage available for plating the heavier coated side. In a typical electrolytic tinning line of the foregoing type, 80,000 amperes obtained from eight 10,000-ampere generators 'are available for coating one side of the steel strip. Each generator is preferably connected to 'four anodes. With the foregoing arrangement, movement of the strip at a speed up to 6001 feet per minute in an operation depositing a 1.00 pound per base box coating is possible. When the strip is traveling at a speed of 600i feet per minute, approximately 27,000' amperes are necessary to produce a 0.25 pound per base box coating of tin on the opposite side of the strip. The latter amperage can, of course, be obtained either by reducing uniformly the current on each of the anodes or by disconnecting certain of the anodes normally employed so that the required amperage is provided by only a few of the electrodes.

In the past, the limited amount of current or amperage required for the lighter coating of tin (i.e., 0.25 pound per base box) usually has been obtained by reducing the current on each of the anodes equally so that the total current applied is the sum required to deposit the desired thickness of tin. In a more recent modification, every other anode used to tin coat the thin side of the strip has -been shut down vfor a short time in rotation so that no anode is down for any prolonged period, as described in U.S. Patent No. 2,723,953. So long as the total amperage applied equals the number of amperes required to deposit the desired amount of tin, it has been considered immaterial how many anodes were used to apply the said current, and lthe amperage on any given anode has been considered of no consequence, provided the anode was not left shut down for a prolonged period.

In the manufacture of tin cans from diiierentially coated `tin plate, Where soldering operations are employed to for-rn side seams or for other purposes, it is very important, especially in high-speed automatic manufacturing techniques, that the tin plate produced have good solderability characteristics and appearance. Frequently, difculties have been encountered with poor appearance and solderability of differentially coated tin plate, and particularly, with lighter weight tin coatings, such as those having a thickness of about 0.25 pound per base box.

It is, therefore, an object of the present invention to provide an improved process for electrolytically plating a thin coating of tin which has improved solderability characteristics on the thin coated side.

Itis a `further object of the present invention to provide an improved process for differentially tin coating a steel strip so as to obtain a product having improved appearance and solderability characteristics on the thin `coated side.

Other objects of the present invention will -be apparent from the vdetailed description and claims to follow.

In the production of differentially coated electrolytic tin plate on an alkaline-type electrolytic tinning line, it has been found that poor appearance and solderability of the thin coated side having sa tin coating of 0.25 pound per base box, or less, results when the anodic current density is maintained below about 2,5 .amperes per square foot in the first portion of the plating bath, and that good appearance and substantially improved solderability are obtained-when the current density of the anodes in the initial portion of the plating `bath is maintained `above a critical current density of 25 -amperes per square foot, and this obtains whether the anodes are in either the filmed or untilmed state.

It has lalso been found that to attain improved results, the light tin coating (i.e., a tin coating having a lWeight of 0.25 pound per base box, or less) should be deposited by consecutive plating -anodes charged with the required plating amperage, and not by having one `anode charged with adequate plating amperage and the next adjacent anode dead or at -a minimal amperage, -or !by having a group of anodes intermediate the ends of the series of charged plating `anodes in the plating bath dead or at just the minimal amperage for maintaining the anode positively charged.

The present invention may best be understood with reference to the accompanying drawing in which:

FIGURE 1 is a fragmentary schematic side elevational view of a tin plating line which can be used in the practice of the present invention; and

FIG. 2 is a graph showing the solderability values obtained with the present invention versus the free tin weight of tin plate production by the present invention and comparable values obtained when certain of the intermediate anodes in the pl-ating bath are dead.

A series of comparative runs were made on an electrolytic tin plate line of the type shown in the drawing for producing differentially coated tin plate having a coating weight of 1.00 pound per base box on the heavier side and 0.25 pound per base box on the lighter side. In each instance, the steel strip in a cathodic state was passed at a line speed of about 600l feet per minute successively through the following steps: pickling, plating, marking, ilow brightening, cathodic cleaning in aqueous sodium carbonate, and a chromic acid dip. The base metal strip 10 consisted of a flat rolled thin gauge low-carbon steel as commonly used in electrolytic tin plating. The electrolytic tinning step was conducted in an aqueous alkaline sodium stannate solution containing 1012 grams per liter of sodium hydroxide and -30 grams per liter of tin at a temperature of 180 F. to 210 F. A current `density on the anodes 11 for plating the said heavier side of about 45 amperes per square foot was employed, and the current density at the anodes 12 used for plating the lightweight side was regulated as hereinafter specified. Each of the anodes 11 and 12 was connected with a generator 13 and preferably with one generator supplying current to four anodes. The generators 13, for convenience, are numbered 1 through '16, with the even numbers designating generators which are connected with anodes plating the thin side of .the strip 10, :and the odd numbered genera-tors connected with the anodes plating the heavier side. Generator No. 2, for example, is connected with the first four anodes plating the light side, and generator No. 4 is connected with the next four anodes plating the light side. The strip 10 is guided over a plurality of laterally spaced upper rollers 14 and lower rollers 15, with the lower rollers 1S being spaced vertically from the upper rollers 14. The strip alternately moves downwardly and then upwardly within the plating -bath or solution contained in the plating tank 16. The 4anodes 11 and 12 are disposed within the tank 16 and alternate anodes 11 are exposed through the plating bath to the underside or heavier side of the strip, and the remaining anodes 12 are exposed to the upper or lighter side of the strip.

For the purpose of evaluating the solderability characteristics of the tin plate, the so-ealled capillary rise method was employed. In accordance with this method, a suitable tin plate sample is taken and is cut to provide a small elongated strip, eg., approximately 3 x 1". This strip is then bent or doubled upon itself until the two halves are spaced only a very slight distance apart, the spacing being regulated by using a jig and anvil to insure theV same configuration and dimensions in successive samples. The bent sample is then immersed to a predetermined depth in molten solder for a fixed .period `and then removed and cooled. The bent or ifolded end of the sample is cut off and the two halves are pulled apart to reveal the extent of rise of solder into the space between the strip halves. Using a scale, the capillary rise is measured to the nearest sixteenth of an inch. For acceptable solderability, tin plate having a coating weight of 0.25 pound per base box should display a minimum capillary rise of W16" (reported simply as 9), and tin plate having a coating yweight of 1.00 pound per base box should display -a minimum capillary rise of 2%6 (reported as 20).

As seen in the following tabulations, the current density of the anodes for plating the light-weight side of the strip was varied in each run by regulating each of the eight generators designated by the even numbers 2 through 16.

Run A Generator: Current density applied 2 26.0 amperes/ft2. 4 26.0 amperes/ft2. 6 22.0 amperes/ft2. 8 22.0 amperes/ft2. 10 18.0 amperes/ft2. 12-14-16 Minimum current to positively charge anodes.

Similar results were obtained with regards to improving appearance and providing high solderability values when the first four generators, which are all of the generators operating on the 0.25 pound per base box side of the strip, had a current density above 25 amperes per square foot and with the anodes in the unlmed state. The anodic current density on each of the generators is shown in the following table.

Generator: Current density applied 2 27.0 amperes/ft2. 4 27.0 yamperes/ft2. -6 31.0 amperes/ft2. 8 27.0 amperes/ft2. 10-12-14-16 Minimum current to positively charge anodes.

Solderability values of 23-30 were obtained for the said light-weight side of the latter strip.

Run Cn In contrast with the foregoing results, are the following results which were produced by reducing the current substantially uniformly in all of the generators.

Current `density applied, Generator: amperes/ft2.

The lighter weight coating produced under the latter tinning conditions wherein the current density of each of the generators connected to the anodes operating on the 0.25 pound per base box (0.25 lb./b.b.) side of the strip was below 25 amperes per square foot, exhibited a very dull, grey appearance and had poor solderability characteristics, as evidenced by solderability values of 13 for the said light-weight side of the strip.

Run "Du A tinning run was conducted in the same manner as in the preceding runs, but with the said generators providing the following applied amperage.

Current density applied From the data in the following table, it will be evi-dent that the average solderability value obtained on a strip with the tin coating lof 0.25 pound per base box applied under the foregoing conditions of intermittent plating was about 13, with a spread of from 0 to 26 in the solderability values. This very wide spread in the solderability values and generally lower solderability values appears to be characteristic of strips tin plated under intermittent plating conditions:

Solderability Values Corresponding t0 Free Tiu (Lbs/bb.)

It will thus be evident that even when the plating anodes are supplied with a current density in excess of 25 amperes per square toot, there is no improvement in the solderability values over that obtained when all the anodes are used at a reduced :current density, if the tin is deposited under intermittent plating condiitons, as t above illustrated.

In FIG. 2 of the drawing, the lower curve therein shows the solderability values and the corresponding free tin values obtained Ifrom four separate electroplating'runs conducted under intermittent plating conditions similar to run D, wherein the anodic current density was in excess of 25 amperes per square foot but with two or more anodes disposed between the plating anodes being supplied with no current or with a minimal amount of current.' The upper curve represents the typical results obtained when electroplating under conditions similar to runs A and 13, which conditions are favorable for producing tin plate having good solderability.

The free tin values dene the weight of tin, in pounds per base box7 in the tin coating which remains unalloyed with iron of the base after the heat treatment thereof during ilow brightening. The total tin coating weight, generally given as pounds per base box, consists of the weight of the free tin plus the weight of the combined tin in the iron-tin alloy layer formed during ilow brightemng.

As shown by the data of run A, very significant-1y improved tin plate is produced when, for example, at least about the rst 50% of the tin coating is plated under high current density conditions wherein the anodic current density is at least 25 amperes per square foot and wherein the said plating is effected by consecutively disposed plating anodes. Thus, as shown in run D, even when in excess of the ii-rst 50% of the tin plate (and in fact 100% of the tin plate) is deposited on the strip at a current density of at least 25 amperes per square foot, relatively poor solderability and appearance result when some of the anodes in the plating bath which are interposed between the plating anodes are dead or at such a very low current density.

The present invention can be practiced by using separate generators of suitable capacity for supplying current to the individual anodes or by having one .generator for a group of anodes, as in `the examples. Except for the critical conditions of the plating current on the initial anodes (e.g. the first 2-4 generators) which are used to deposit about the initial 50% of the thin coating of tin and the necessity of using consecutively disposed plating anodes for depositing the thin coating, the electrotinning operations are conventional and `are conducted in the usual manner; that is, a strip of annealed low-carbon steel is passed through equipment arranged in sequence wherein it is cleaned or pickled, rinsed, plated, rinsed and dried, subjected to sufcient heat to melt the coating deposited thereon, quenched, dried and oiled, after which it may be coiled or cut up into sheets of desired length. These operations and the equipment for carrying them out are well known and have not been described in detail.

Others may practice the invention in any of the numerous ways which are suggested to one skilled in the art, by this disclosure, and all such practice of invention are considered to be a part hereof which fall within the scope of the appended claims.

I claim:

l. In a method of continuously ditierentially electrolytically tinning a metal strip in -a tin plating bath to deposit a light-weight tin coating on one side having a weight of not substantially in excess of about 0.25 pound per base box while simultaneously depositing a substantially heavier weight tin coating on the other side which includes passing said strip continuously through a tin plating bath having two series of anodes disposed therein with each series of anodes being substantially equal in number and connected with source means of electric current with one of said series of anodes in the said bath being adapted to deposit tin on the lightweight side of the strip and the other series of anodes in the said bath being adapted to deposit tin on the heavy-weight side of the strip, the irnprovemeut comprising; maintaining only certain consecutive plating anodes of said one of said series at an anodic current density sucient to deposit tin on the light-weight side of said strip, said certain consecutive plating anodes being less than the total number of anodes of said one of said series, with the remaining anodes of said one of said series having at most only a minimum current to positively charge the anodes, and maintaining at least the initial consecutive plating anodes of said one of said series at an anodic cur-rent density in excess of 25 amperes per square foot, all of the tin coating on said light-weight side being deposited by said consecutive plating anodes.

2. In a method of continuously diiierentilly electrolyt- -ically tinning a metal strip in an alkaline tin plating bath to deposit a light-weight tin coating on one side having a weight of not substantially in excess of about 0.25 pound per base box while simultaneously depositing a substantially heavier weight tin coating on the other side which includes passing said strip continuously through an alkaline tin plating bath having two series of anodes disposed therein with each series of anodes being substantially equal in number and connected With source means of electric current with one of said series of anodes in the said bath being adapted to deposit tin on the lightweight side of the strip and the other series of anodes in the said bath being adapted to deposit tin on the heavy- Weight side of the strip, the improvement comprising; maintaining only certain consecutive plating anodes of said one of said series at an Ianodic current density sucient to deposit tin on the light-weight side of said strip, said certain consecutive plating anodes being less than the total number of anodes of said one of said series, with the remaining anodes of said one of said series having at most only a minimum current to positively charge the anodes, and maintaining at least the initial consecutive plating anodes of said one of said series while depositing atleast about the rst 50 percent of said light-weight side tin coating at an anodic current density having a range between about and 40 amperes per square foot, all of the tin coating on said light-weight side being deposited by said consecutive plating anodes.

3. A method of continuously diterent-ially electrolytically tinning a metal strip as in' claim 1, wherein the said tin plating bath is comprised of sodium stannate.

4. A method of continuously diiferentially electrolytically tinning a metal strip as in claim 1, wherein the said strip is passed through said plating bath at a rate of about 400-800 feet per minute while depositing a tin coating on the heavier weight side of the strip having a thickness of about 1.00 pound per base box and the anodes depositing tin on the heavy-weight side having an operative anodic current density of about 45 amperes per square foot and depositing la tin coating on the lightweight side of the strip having a thickness of about 0.25 pound per base box by means of consecutive plating anodes having an anodic current density of about 25 amperes per square foot.

5. In a `continuous electrolytic tinning process for differentially coating a `ferrous metal strip with a lightweight tin coating having a thickness of not substantially in' excess of about 0.25 pound per base box on one side while simultaneously depositing a substantially heavier weight tin coating of at least about one pound per base box on the other side and in which the metal stript is tinned by continuously passing the said strip in a cathodic state between two series of spaced anodes disposed in a sodium stannate tin plating bath with each series of 'anodes being substantially equal in number and connected with source means of electric current for maintaining a desired current density on the anodes, with one of said series of anodes being adapted to deposit tin on the light-weight side of the said strip and the other of said series being adapted to deposit itin on the heavier weight side of the said strip, the improvement comp r`is ing; maintaining only certain consecutive plating anodes of said one of said series at an anodic current densityv suicient to deposit tin on the light-weight side of said strip, said certain consecutive anodes being less than the total number of anodes of said one of said series, with the remaining anodes of said one of said series having at most only a minimum current to positively charge the anodes, and maintaining at least the initial consecutive plating anodesl of said one of said series while depositing at least about the rst 50 percent of said light-weight side tin coating at an anodic current density in excess of 25 amepres per square foot, `all of the tin coating on said 8 light-weight side being deposited by said consecutive plating anodes.

6. In a continuous electrolytic tinning process for differentially coating a metal strip with a light-weight tin coating having a thickness of about 0.25 pound Vper base -box on one side while simultaneously depositing a substantially heavier weight tin coating of at least about one pound per base box on the other side and in which the metal strip is tinned by continuously passing the'said strip in a cathodic state between two series of spaced anodes substantially equal in number and disposed in a sodium stannate tin plating bath with each anode being connected with a source of electric current for maintaining a desired current density thereon with one of said series of anodes being adapted to deposit tin on the light-weight side of the said strip and the other of said series being adapted to deposit tin on the heavier weight side of the `said strip; the improvement comprising; maintaining only certain consecutive plating anodes of said one of said series at an anodic current density sufficient to deposit tin on the light-weight side of said strip, said certain consecutive plating anodes being less than the total number of anodes of said one of said series, with the remaining anodes of said one of said series having at most only a minimum current to positively char-ge the anodes, and maintaining at least the initial consecutive plating anodes of said one of said series While depositing at least about the first 50 percent of said light-weight side tin coating at an anodic current density having a range between about 25 and 40 amperes per square foot, all of the tin coating on said light-weight side being deposited by said consecutive plating anodes.

References Cited in the le of this patent UNITED STATES PATENTS 2,473,918 Stoltz etal. June 21, 1949 2,494,852 Winterhalter et al Jan. 17, 1950 2,657,177 Rendel Oct. 27, 1953 2,676,146 Rendel Apr. 20, 1954 2,723,953 Burgemeister Nov. :15, 1955 OTHER REFERENCES Principles of Electroplating and Electroforming, 3rd edition, 1949 by Blum and Hogaboom, McGraw-Hill Book Co. Inc., New York, N.Y., pages 327-28.

Gray: Modern Electroplating, Wiley and Sons, New York, 1953, pages 394-5.

Lowenheim: Metal Finishing, vol. 49, No. 3, page (March 1951). 

1. IN A METHOD OF CONTINUOUSLY DIFFERENTIALLY ELECTROLYTICALLY TINNING A METAL STRIP IN AN TIN PLATING BATH TO DEPOSIT A LIGHT-WEIGHT TIN COATING ON ONE SIDE HAVING A WEIGHT OF NOT SUBSTANTIALLY IN EXCESS OF ABOUT 0.25 POUND PER BASE BOX WHILE SIMULTANEOUSLY DEPOSITING A SUBSTANTIALLY HEAVIER WEIGHT TIN COATING ON THE OTHER SIDE WHICH INCLUDES PASSING SAID STRIP CONTINUOUSLY THROUGH A TIN PLATING BATH HAVING TWO SERIES OF ANODES DISPOSED THEREIN WITH EACH SERIES OF ANODES BEING SUBSTANTIALLY EQUAL IN NUMBER AND CONNECTED WITH SOURCE MEANS OF ELECTRIC CURRENT WITH ONE OF SAID SERIES OF ANODES IN THE SAID BATH BEING ADAPTED TO DEPOSIT TIN ON THE LIGHTWEIGHT SIDE OF THE STRIP AND THE OTHER SERIES OF ANODES IN THE SAID BATH BEING ADAPTED TO DEPOSIT TIN ON THE HEAVY-WEIGHT SIDE OF THE STRIP, THE IMPROVEMENT COMPRISING; MAINTAINING ONLY CERTAIN CONSECUTIVE PLATING ANODES OF SAID ONE OF SAID SERIES AT AN ANODIC 